MC–19

MC–3

MC–4 MC–5

MC–6

MC–10 MC–11 MC–12 MC–13

MC–1

MC–18 MC–20 MC–21

MC–25

MC–26 MC–27

MC–28

MC–30

300°

300°

60°

60°

0°

30°

65°

270° 315° 0° 45° 90°

–30°

–65°

0°

30°

65°

–30°

–65°

2013

U.S. Department of the Interior

U.S. Geological Survey

Prepared for the National Aeronautics and Space Administration

Geologic Map of MTM –15027, –20027, –25027, and 

–25032 Quadrangles, Margaritifer Terra Region of Mars

By Rossman P. Irwin, III, and John A. Grant

Pamphlet to accompany

Scientific Investigations Map 3209
i

Contents

Introduction and Background......................................................................................................................1

Mapping Methods and Data ........................................................................................................................1

Age Determinations.......................................................................................................................................2

Regional Geology...........................................................................................................................................2

Stratigraphy ....................................................................................................................................................4

Noachian Period ...................................................................................................................................4

Hesperian Period ..................................................................................................................................4

Amazonian Period.................................................................................................................................6

Structural Features........................................................................................................................................6

Conclusions.....................................................................................................................................................6

Pre-Noachian Period ...........................................................................................................................6

Noachian Period ...................................................................................................................................6

Hesperian Period ..................................................................................................................................6

Amazonian Period.................................................................................................................................7

References Cited............................................................................................................................................7

Table

1. Crater counts for spatially extensive map units. ......................................................................10
1

Introduction and Background

Mars Transverse Mercator (MTM) quadrangles –15027, 

–20027, –25027, and –25032 (lat 12.5o–28o S., long 330o–335o 

E. and lat 22.5o–28o S., long 324.5o–330o E.; figs. 1, 2) in southwestern Margaritifer Terra include diverse erosional landforms, 

sedimentary deposits, and tectonic structures that record a long 

geologic and geomorphic history. The northeastern regional 

slope of the pre-Noachian crustal dichotomy (as expressed 

along the Chryse trough) and structures of the informally named 

Middle Noachian or older Holden and Ladon impact basins 

dominate the topography of the map area (Schultz and Glicken, 

1979; Schultz and others, 1982). A series of mesoscale outflow 

channels, Uzboi, Ladon, and Morava Valles, integrated these 

formerly enclosed basins by overflow and incision around 

the Noachian/Hesperian transition, although some flooding 

may have occurred earlier (for example, Florenski and others, 

1975; Pieri, 1980; Boothroyd, 1982; Parker, 1985, 1994; Grant, 

1987, 2000; Grant and Parker, 2002). The area includes excellent examples of Late Noachian to Hesperian valley networks 

(Baker, 1982; Carr, 1996), dissected crater rims, alluvial fans, 

deltas, and light-toned layered deposits (LTLDs), particularly in 

Holden and Eberswalde craters (for example, Malin and Edgett, 

2000, 2003; Grant and Parker, 2002; Moore and Howard, 2005; 

Pondrelli and others, 2005, 2008; Lewis and Aharonson, 2006; 

Grant and others, 2008). Structural forms include Tharsis-radial 

grabens, Hesperian wrinkle ridges, floor-fractured impact craters, and severely disrupted chaotic terrains (for example, Grant, 

1987; Chapman and Tanaka, 2002). These well-preserved landforms and sedimentary deposits represent multiple erosional 

epochs and discrete flooding events, which provide significant 

insight into the geomorphic processes and climate change on 

early Mars.

Previous mapping of the area at smaller map scales 

includes the 1:5,000,000 map by Saunders (1979) and the 

1:15,000,000 map by Scott and Tanaka (1986), based on 

Mariner 9 and Viking Orbiter data, respectively. Saunders 

(1979) observed that Holden crater was unusually deeply eroded 

for a stratigraphically young crater with relict secondaries, and 

he interpreted the interior fill as fluvial deposits modified by 

wind. His 1:5,000,000 map also noted mountainous materials 

that were remnants of impact structures and smooth plains of 

aeolian or volcanic material in the Ladon impact basin. Both 

this and the later 1:15,000,000 map recognized the area’s major 

channels and chaotic terrains. Scott and Tanaka (1986) subdivided the highland surface into hilly, cratered, and subdued 

cratered units, and they mapped Holden as a superimposed 

impact crater with aeolian or volcanic fill. Parker and Pieri 

(1985a,b) completed a geologic/geomorphic map of the region 

and identified many of the basic units and structures identified 

here, although they lacked the resolution to subdivide basinfloor deposits. Grant (1987) mapped drainage divides, valleys, 

impact craters, and faults in the Margaritifer Sinus quadrangle 

to constrain its geomorphic history. He described the influence of degraded craters and other topographic features on the 

development of fluvial landforms. Pondrelli and others (2005, 

2008) mapped and interpreted stratigraphic units in Holden 

and Eberswalde craters. In Holden crater, Pondrelli and others 

(2005) identified deposits equivalent to the layered, Holden 

central, and fan 2 units described below, but their interpretations differ from ours in several respects. They interpreted 

the western alluvial bajada as mass-wasting materials, did not 

identify the crater’s central ring structure, and interpreted bouldery deposits and eroded terrain emerging from Uzboi Vallis as 

having a glacial rather than flood origin. Pondrelli and others 

(2008) interpreted three depositional sequences in Eberswalde 

crater that could not be defined at the scale of our map, and 

Rice and others (2011, 2013) mapped small outcrops and structures in Eberswalde. Other recent papers focused on the map 

area’s major sedimentary deposits, which are discussed below 

(Malin and Edgett, 2000, 2003; Grant and Parker, 2002; Moore 

and others, 2003; Jerolmack and others, 2004; Bhattacharya and 

others, 2005; Moore and Howard, 2005; Pondrelli and others, 

2005, 2008; Lewis and Aharonson, 2006; Wood, 2006; Grant 

and others, 2008; Milliken and Bish, 2010). These four quadrangles are part of a series of 1:500,000- to 1:1,000,000-scale maps 

in Margaritifer Terra sponsored by the National Aeronautics 

and Space Administration’s Planetary Geology and Geophysics 

Program (Fortezzo and others, 2008; Grant and others, 2009; 

Williams, 2010).

Mapping Methods and Data

Our photogeologic mapping approach delimited unconformity-bounded geologic units rather than units based only on 

secondary erosional or structural landforms (Salvatore, 1994; 

Tanaka and others, 2005). In some cases, significant outcrops 

such as LTLDs are <2 km across (2 mm at map scale), so we 

grouped them with a more extensive cap rock, although it 

was uncertain whether the LTLDs and cap rock have the same 

lateral extent. Unit names are descriptive where unique primary 

morphology was evident (for example, fan 2 unit, chaotic unit, 

crater 1 unit, dune unit), but basin-filling materials of more 

ambiguous origin and composition have unit names based on 

their geographic and stratigraphic location (for example, the 

basin fill 1 unit is older than the basin fill 2 unit).

The U.S. Geological Survey provided the 231 m/pixel 

base map derived from the Viking Mars Digital Image Mosaic 

2.1. The base uses a transverse Mercator projection, the Mars 

Orbiter Laser Altimeter (MOLA) 2000 datum, and a central meridian of 330o E. We made an equivalent base mosaic 

using Mars Odyssey Thermal Emission Imaging System 

(THEMIS) daytime infrared (DIR) imaging at 100 m/pixel 

(fig. 2), and we reprojected and georeferenced part of the 

Arizona State University THEMIS nighttime infrared mosaic 

2.0 at 231 m/pixel as an overlay (fig. 3). Detailed mapping 

was based on THEMIS visible images at 18 m/pixel and Mars 

Reconnaissance Orbiter (MRO) Context Camera images at 

5 m/pixel. Where available, Mars Orbiter Camera (MOC) 

images at ~2–5 m/pixel, MRO High Resolution Imaging 

Science Experiment (HiRISE) visible images at 25 cm/pixel, 

and Compact Reconnaissance Imaging Spectrometer for Mars 

(CRISM) products facilitated local interpretations of within-unit 
2

stratigraphy. Topographic measurements were based on the 128 

pixel/degree MOLA Mission Experiment Gridded Data Record. 

Using ArcGIS 9.3, we drafted line work at 1:250,000 scale for 

publication at 1:1,000,000 scale. Unit contacts have a streamdigitized vertex spacing of ~500 m and are attributed as definite 

or approximate. Linear features include faults, crater rims, and 

small valleys.

Age Determinations

We constrained age relations using a count of all primary 

craters >1 km in diameter, as seen in THEMIS DIR imaging 

of the map area (some secondary craters 1–2 km in diameter 

may have inadvertently been included, particularly near Holden 

crater). We did not exclude any surfaces from the counts for 

each unit. The resulting relative ages (table 1) are consistent 

with observed superposition and crosscutting relations, within 

the error bars of the crater counts. Unit ages are reported as 

N(x), the number of craters greater than x km in diameter per 

106 km2, plus or minus the square root of the count, also normalized to 106

 km2. The geologic epoch associated with each count 

and range of epochs covered by the error bars are also given 

(Werner and Tanaka, 2011). Isochron plots using the Hartmann 

(2005) method are provided for large surfaces including the 

entire map area, the terra unit, and the Holden crater rim and 

ejecta. These plots show model absolute ages for major resurfacing events, listed oldest to youngest:

•	 Ladon and Holden basin impacts (prior to ~3.7–3.9 Ga)

•	 decline of ubiquitous crater degradation and loss of 

small craters (3.5–3.7 Ga)

•	 Holden crater impact (~3.0–3.5 Ga)

These model ages highlight the uncertain timespan over 

which most of the landscape degradation occurred, but prolonged crater degradation with at least intermittent fluvial 

erosion appears to have modified the Martian surface over 

hundreds of millions of years.

Regional Geology

The map area’s diverse geologic units formed during 

three to four distinct epochs of erosion and resurfacing, which 

include two relatively brief but important flooding events 

(Grant, 1987). Geophysical and isotopic evidence suggests that 

the crust formed within the first 50 m.y. of Martian history. The 

isostatically compensated long-wavelength topography of the 

crustal dichotomy dates to the pre-Noachian Period, before the 

relict cratering record was established (Solomon and others, 

2005; Nimmo and Tanaka, 2005; Andrews-Hanna and others, 

2008; Irwin and Watters, 2010). Early loading of the lithosphere 

by Tharsis, located north of the map area, may be responsible 

for the Chryse trough, which is visible across the map area 

(Phillips and others, 2001). Basin and ring structures of the 

Middle Noachian or older Ladon and superimposed Holden 

impact basins have survived to the present (Schultz and others, 

1982). The inner/outer ring diameters are 500/1,000 km and 

300/600 km for Ladon and Holden impact basins, respectively. 

These structures and their associated topography controlled the 

planform of later flood channels and smaller fluvial valleys (for 

example, Parker, 1985; Grant, 1987; Grant and Parker, 2002), 

which are mostly radial to basin centers or follow the basins’ 

outer rings (fig. 2).

The first time interval of landscape degradation was concurrent with heavy meteorite bombardment (Grant, 1987), such 

that larger, older craters are more heavily degraded than are 

smaller, younger ones (Craddock and Maxwell, 1993). The suite 

of physical processes involved in Noachian degradation tended 

to smooth the high-frequency relief in the highland landscape, 

including a reduction in the population of small (<30 km diameter) impact craters (for example, Neukum and Hiller, 1981; 

Strom and others, 1992), rim erosion and infilling of larger craters (for example, Forsberg-Taylor and others, 2004), and retreat 

of promontories and scarps. In this map area, few Noachian craters <8 km in diameter are preserved (figs. 4–6). As seen across 

much of Mars, the resulting landscape was never integrated into 

mature drainage basins (for example, Pieri, 1980; Stepinski and 

Stepinski, 2005; Luo and Howard, 2005), and it is undersaturated with both small craters and small valleys, suggesting that 

neither impact cratering nor fluvial erosion achieved the dominance found on the Moon and Earth, respectively. Relatively 

high infiltration capacities that helped to limit runoff may have 

further inhibited valley incision (Grant, 1999, 2000). Irwin and 

others (2011) interpreted the Noachian highlands as a transportlimited landscape, where weathering outpaced weak and (or) 

intermittent fluvial erosion until near the Noachian/Hesperian 

transition, allowing long-term reduction of relief without substantial incision of valleys. Crater size-frequency distributions 

suggest that this early epoch of landscape degradation had two 

definable components (Grant, 1987). Eberswalde crater formed 

and was partially degraded during this time.

Incision of branching valleys characterized the second time 

interval of erosion at approximately the Noachian/Hesperian 

transition. Howard and others (2005) suggested possible causes, 

including an increase in the magnitude and (or) frequency of 

precipitation events, a reduction in the weathering rate and sediment supply, or formation of a duricrust that facilitated incision 

by reducing the infiltration capacity and (or) sediment supply. 

The declining impact flux and high rates of volcanic activity 

in the Late Noachian to Early Hesperian epochs (for example, 

Greeley and Schneid, 1991) may have facilitated atmospheric 

thickening around that time.

Incision of the Uzboi, Ladon, and Morava Valles mesoscale outflow channels represents a significant erosional event 

in the Late Noachian to Early Hesperian epochs (table 1). The 

slope of the crustal dichotomy/Chryse trough controlled the 

overall flow path, which was locally reoriented radial to the 

Holden and Ladon impact basins (figs. 1, 2). Uzboi Vallis heads 

at the incised margin of the Argyre impact basin and delivered 

enough water to overflow both the Holden and Ladon impact 

basins (Parker and Pieri, 1985c). The multiple anabranching 
3

outlet channels of Ladon and Morava Valles reflect rapid 

flooding of the basins and a high discharge that the pre-valley 

topography could not confine into a single channel. The only 

evident source for these large flows is Argyre basin (Parker, 

1985, 1994), which may have collected meltwater from a contemporary southern polar ice cap (Clifford and Parker, 2001). 

The topographically defined contributing watershed extended 

far to the south of Argyre and encompassed as much as 9 

percent of the surface of Mars (Banerdt, 2000). Flow appears to 

have continued through the informally named Margaritifer basin 

to the northern lowlands via Mawrth and, perhaps, Ares Valles 

(Grant and Parker, 2002; Rotto and Tanaka, 1995; Irwin and 

Grant, 2009). This is the first and larger of two discrete erosional events driven by water that was exotic to the map area. 

It also represents a rare but high-volume return flow of surface 

water from high southern latitudes to the lowlands, independent 

of slower atmospheric or subsurface pathways. The Martian 

highlands maintained a multibasin landscape throughout the 

Noachian period, and few channels were established for surface 

drainage from enclosed southern basins.

Holden and (the stratigraphically younger) Ostrov craters formed in the Hesperian period, based on crater counts 

(table 1). Their ejecta morphology is subdued, but their central 

peaks and secondary craters are still visible, particularly north 

of the craters’ continuous ejecta. Holden crater interrupted 

distal Uzboi Vallis (fig. 1), where it had debouched into the 

Holden impact basin (Bond and Hale craters also overlie Uzboi 

Vallis near its head, outside of the study area; fig. 1). Holden 

crater ejecta mostly buried the older Eberswalde crater to the 

north. Following the Holden and Ostrov impacts, LTLDs and 

coarse-grained alluvial fans accumulated in the craters over an 

unknown period of time.

A Hesperian flooding event postdates deposition of LTLDs 

in Holden crater. The Holden crater rim had dammed Uzboi 

Vallis, forming an enclosed basin within the valley between the 

rims of Holden and Bond craters. The low point of the basin’s 

divide was the Holden crater rim at –350 m. A lake formed 

in Uzboi Vallis from sources on the surrounding cratered 

surface and (or) from its Nirgal Vallis tributary, accumulating phyllosilicate-bearing LTLDs (fig. 7) like those found in 

Holden and Eberswalde craters (Grant and others, 2010). A 

phyllosilicate-bearing layer occurs near the top of the section 

exposed in Nirgal Vallis southwest of the map area (Buczkowski 

and others, 2010), and it may provide a concentrated source 

for these phyllosilicates. This lake overflowed and incised the 

Holden crater rim, initially forming multiple channels (fig. 8) 

before flow coalesced into one entrance breach, draining the 

lake almost entirely and incising an inner valley 30 km up the 

Uzboi Vallis floor (Grant and Parker, 2002; Grant and others, 

2008). The multiple overflow points again suggest a substantial 

discharge into and from the 4,000-km3

 Uzboi lake, and the inner 

valley shows that discharge through Uzboi Vallis somewhat outlasted drawdown of the lake. However, flow through the system 

was not geologically long-lived, because the Uzboi Vallis floor 

is not fully dissected, and deposits in Holden crater related to 

the breach were not incised or reworked (fig. 8). If any deposits 

accumulated in Nirgal Vallis during the infilling of Uzboi Vallis, 

they were eroded during and after drawdown of the lake. These 

observations are consistent with a geologically brief episode of 

infilling and breaching in Uzboi Vallis.

A possible alternative scenario involves initially slow 

drainage from Uzboi Vallis into Holden crater (over or through 

the fractured crater rim) for a period of time before the rim 

breach and flooding occurred. This source of clear water could 

have maintained a paleolake in Holden crater long enough for 

LTLDs to accumulate from erosion of the crater walls. Finally, 

a positive feedback between rim erosion and discharge resulted 

in catastrophic drainage of the Uzboi lake into Holden crater. 

A similar delayed outflow occurred at paleolake Bonneville in 

the western United States, where the basin’s outlet (an alluvial 

fan at Red Rock Pass, Idaho) maintained the lake level at the 

Bonneville high stand for some time before a catastrophic incision and flood (O’Connor, 1993). In this scenario, the angular 

unconformity between LTLDs and overlying coarse-grained 

stratigraphy in Holden crater (all within the layered unit, Hl, or 

between the layered unit and the fan 1 unit, Hf1) would represent very little time.

Alluvial fans continued to develop in Holden crater for 

some time after the Uzboi flood, although perhaps under drier 

conditions. The southern end of the Holden bajada overlies the 

termini of flood channels from the Uzboi rim breach. Floor fractures in Holden crater crosscut deposits from Uzboi Vallis, but 

they do not crosscut alluvial fans that they intersect, also suggesting that fan activity extended beyond the Uzboi flood. Grant 

and Wilson (2011) dated the end of alluvial fan development 

to the Amazonian period, perhaps as early as the Hesperian/

Amazonian boundary. It is unclear whether the Eberswalde 

delta was contemporary with flooding from Uzboi Vallis into 

Holden crater or with later fan activity, but the alluvial fans in 

other topographically isolated craters in the region suggest that 

fan development was a regional occurrence and not dependent 

on water from Uzboi Vallis. The Eberswalde delta was not 

deeply entrenched by its contributing stream as the lake level 

fell, suggesting that fluvial erosion declined abruptly relative 

to Pleistocene/Holocene climate change on Earth (Irwin and 

others, 2005). The multibasin highland plateau may have accelerated this decline by topographically and thermally trapping 

water (Irwin and others, 2011). Subsequent erosion was primarily aeolian and most effective on friable deposits, including 

LTLDs and interchannel fines on alluvial fans. The preservation 

of Late Noachian to Hesperian landforms on Mars attests to the 

severe decline in weathering and erosion rates around that time 

(Craddock and others, 1997; Golombek and others, 2006).

Late Hesperian to Early Amazonian extensional faults and 

chaotic terrains are concentrated on basin surfaces as well as 

in Ladon and Morava Valles. The chaotic terrains are mostly 

located within and may have been influenced by older multiringed impact basin structures.
4

Stratigraphy

Noachian Period

The oldest map unit is the Middle Noachian (or perhaps 

older) mountainous unit (Nm), composed of isolated bedrock 

promontories with Noachian or older crystallization ages and 

exposed as fractured ring structures during the Ladon and 

Holden basin impacts (Schultz and others, 1982). Noachian 

landscape degradation reduced these mountains in size and 

created fringing pediments or deposits of terra unit material. 

The mountainous unit also includes the central peak structures 

of Holden and Ostrov craters, which are interpreted to similarly 

expose deep crustal rocks.

The most widespread unit is the Late Noachian to Early 

Hesperian terra unit (HNt), which includes a diverse and 

spatially variable mix of fractured basaltic bedrock (Bandfield 

and others, 2000), impact ejecta, weathered rocks (Murchie and 

others, 2009), fluvially reworked sediments, airfall or aeolian traction materials, and perhaps other types of rocks. The 

unit is interpreted as a geomorphic surface composed of both 

erosional and depositional outcrops, and it cannot be meaningfully subdivided (Tanaka and others, 2005). Fluvial dissection 

is common, particularly on slopes, and stratification is evident 

in some places. Terra unit surfaces are typically rolling and do 

not retain small craters well, relative to many smooth basin 

fills (Malin, 1976; Malin and Edgett, 2001). Impact gardening 

during and after the Noachian period has substantially reworked 

these surfaces (Hartmann and others, 2001). Noachian landscape-degradation processes reworked the interior slopes of the 

Middle Noachian or older Ladon and Holden impact basins, so 

these areas are included in the terra unit. Superimposed craters 

suggest that substantial crater degradation continued into the 

Early Hesperian epoch in the map area (table 1).

Hesperian Period

Late Noachian to Early Hesperian channel materials of 

Uzboi, Ladon, and Morava Valles include exposed bedrock 

outcrops, eroded surfaces of the terra unit, and veneers of 

alluvial sediment. The channel 1 unit (HNch1) represents 

initial dissection of the surface; these surfaces were abandoned 

when flow coalesced and downcut the main channels (HNch2), 

which conveyed any later flows out of Argyre basin. The eroded 

morphology and widespread, presumably alluvial, surfaces in 

the channel 1 unit distinguish it from the terra unit. Based on the 

>1-km crater population, the HNch1 and HNch2 units do not 

have distinguishable ages (N(1) = 3,959±534 and 3,983±996, 

respectively), and Holden and Ostrov secondary craters in the 

area complicate counts at smaller diameters. However, crude 

layering in the Ladon impact basin seen in meter-resolution 

imaging (fig. 9) suggests multiple pulses of discharge into the 

basin (Grant and Parker, 2002).

In Holden impact basin, Holden crater ejecta partially 

resurfaced an older fill unit (basin fill 1 unit, HNb1), and the 

sequence subsequently developed a knobby morphology. Later 

sedimentary deposits and recent aeolian sand have partially 

infilled this rough surface (fig. 10). The Holden impact basin 

also contains a superimposed thick fill unit (basin fill 2 unit, 

HNb2) that appears similar to fill materials confined to other 

impact craters in the area (smooth plains that retain small 

superimposed craters). It is interpreted as moderately to strongly 

indurated alluvial, volcanic, or aeolian materials; origin and 

composition may vary within and between individual basins.

Contemporary units in Ladon basin include the basin fill 2 

(HNb2) and basin fill 3 (Hb3) units. Higher-standing HNb2

surfaces uplifted by compressional faulting consist of a partly 

stripped, darker-toned surface layer and underlying mediumto lighter-toned materials (fig. 11), which overlie the crudely 

stratified deposit mentioned above. The mechanical weakness 

and rugged texture of the stripped and lighter-toned materials 

are consistent with moderately indurated sedimentary deposits 

but not basaltic lava flows. The lighter-toned layer may represent fine-grained aqueous sediments, evaporites, or a duricrust, 

which are all consistent with a large arid-zone basin. The Hb3

unit embays the HNb2 unit, is darker in tone, preserves a dense 

population of small impact craters, and has a higher nighttime 

brightness temperature. The crater counts (table 1) for these two 

units collectively are similar to those for the Uzboi, Ladon, and 

Morava flood and to the end of widespread crater degradation 

in the area. The exposed stratigraphy in Ladon basin suggests multiple facies and depositional environments, which we 

interpret to consist of coarse flood deposits from Ladon Vallis, 

finer-grained alluvium from radial valley networks (both part of 

HNb2), and embayment by volcanic materials or perhaps wellindurated sedimentary deposits (Hb3). 

The Hesperian ejecta of Holden and Ostrov craters 

(Hc1) postdate all of the units above and resurfaced much of 

the southern third of the map area. These ejecta and secondary craters were then modestly degraded and dissected. The 

timing of these two impacts is confirmed by their mutually 

consistent crater counts, superposition of Holden crater on 

Uzboi Vallis, observation of Holden crater secondaries on the 

Ladon impact basin floor deposits, and superposition of Ostrov 

secondaries over those of Holden crater. Megabreccia from the 

Holden crater impact is exposed along the southern crater wall 

(Grant and others, 2008). A resistant unit on the Holden and 

Eberswalde crater floors (He), where overlying sedimentary 

deposits have mostly been removed (although small stratified 

outcrops remain), may be composed of materials related to the 

impact or early infilling deposits.

After the Holden crater impact, Holden and Eberswalde 

craters accumulated thinly bedded, phyllosilicate-bearing 

LTLDs (Lewis and Aharonson, 2006; Grant and others, 2008; 

Milliken and Bish, 2010) (fig. 12), suggesting a similar depositional environment. The largest outcrops of these rocks occur 

where they are exposed beneath a more resistant capping material; this cap rock defines the mapped extent of the layered unit 

(Hl). In Holden crater, the thickness of beds in at least the upper 

part of the section is less than a meter (it is difficult to define 

bed thickness in some portions of the lower and middle LTLD 

members in Holden crater, fig. 13), and beds are laterally traceable for as much as kilometer distances. The deposits are generally confined to low, common elevations within the craters and 
5

do not drape high-standing surfaces (except for some breached 

intercrater basins west of Eberswalde), favoring low-energy 

alluvial or lacustrine deposits rather than airfall materials or volcanic ash. The deposits are weakly indurated and susceptible to 

aeolian erosion, consistent with a fine grain size. Alluvial fans 

and paleochannels overlie the LTLDs in both craters, consistent 

with aqueous deposition.

The origin of phyllosilicates observed in LTLD outcrops 

(part of units He and Hl) of Holden crater, Eberswalde crater, 

Ladon impact basin, Uzboi Vallis, and other depressions has 

important implications for habitability (Grant and others, 2008; 

Milliken and Bish, 2010). To account for landscape degradation, 

the highland surface must have undergone prolonged chemical 

and mechanical weathering during heavy impact bombardment. 

Flowing water may have reworked phyllosilicates from weathered upland surfaces and transported them into depositional 

basins, although some phyllosilicates were likely produced by 

weathering of the basin walls. Holden crater drained internally 

for most of its erosion, so phyllosilicates were either eroded 

from sedimentary rocks exposed on the crater walls, or substantial chemical weathering occurred within the crater (Grant and 

others, 2008; Milliken and Bish, 2010), possibly due in part to 

impact-related hydrothermal alteration (Newsom and others, 

1996; Schwenzer and Kring, 2009).

Other stratified deposits in Holden crater are the fan 1 unit 

(Hf1) and the Holden central unit (Hh). The fan 1 unit is coarse 

grained with many boulders visible in HiRISE imaging, despite 

the fan’s low gradient. Its coarse bedding has steeper dips and 

truncations near the mouth of Uzboi Vallis, grading to more 

planar bedding toward the center of the crater (Grant and others, 

2008). The fan has a multi-lobed planform emanating from 

Uzboi Vallis into the crater. These characteristics suggest origin 

during a large flood when a paleolake in Uzboi Vallis breached 

the Holden crater rim (Grant and Parker, 2002). One lobe was 

oriented toward a gap in Holden crater’s inner ring, which confines much of the Holden central unit (Hh). Unit Hh is defined 

by its low albedo, low thermal inertia, and overall smooth morphology with some rugged outcrops of underlying material. The 

Holden central unit is interpreted as fine-grained, low-energy 

deposits that have been substantially etched and mostly buried 

by a recent aeolian sand sheet (Pondrelli and others, 2005).

Holden and Ostrov craters include some of the most 

extensive, best-preserved alluvial fans on Mars (AHf2). Large 

alluvial fans formed preferentially along the rims of deep, Late 

Noachian to Hesperian impact craters that were not substantially 

modified by the earlier crater degradation (Moore and Howard, 

2005). Ostrov crater has unusually deep rim dissection around 

the entire crater wall, sourcing an alluvial bajada that spans the 

circumference of the crater (fig. 14). These fans ramp down to 

the central peak, which is fringed by a small area of low-relief 

stratified deposits. The lack of evidence for deep ponding suggests that the groundwater table in the area was typically low 

or below the crater floor, consistent with infrequent runoff in an 

arid Hesperian paleoclimate.

Holden crater contains a fringing bajada along its west 

wall, but the east wall experienced very little erosion or deposition. Having formed on the inner ring of the older Holden 

impact basin, Holden crater’s west rim stands higher than 

its east rim. It is possible that an orographic effect enhanced 

precipitation from easterly winds, although orographic enhancement by the crater itself was not required at Ostrov, and other 

craters with fans show no consistent relation between the fan 

orientation and regional air flow (Kraal and others, 2008). 

Alternatively, the greater rim dissection may be due to the 

larger relief and surface area of the slope, enhancing the erosive 

power of runoff to above some threshold for incision or gravel 

transport. The bajada has a relation between contributing area 

and slope that is consistent with fluvial rather than debris-flowdominated fans on Earth, and the inverted channels common on 

fan surfaces show that interchannel deposits were dominantly 

composed of sand and fines (Moore and Howard, 2005), so 

high-magnitude runoff by terrestrial standards may have been 

uncommon or absent at these sites. The inverted paleochannels 

reflect aeolian deflation, or delicate distributary patterns would 

not be preserved, so a supply of fines was required either from 

the source alcoves or less likely from in-situ weathering.

The putative delta at Eberswalde crater has several features 

in common with alluvial deposits elsewhere in the map area. 

The Eberswalde delta was sourced from a drainage basin to 

the west, which had been resurfaced by Holden crater ejecta 

(Malin and Edgett, 2003; Moore and others, 2003; Pondrelli and 

others, 2008). The alluvium (fig. 15) likely consists of Holden 

crater ejecta, weathered rocks from the subjacent terra unit, and 

materials from the dissected Eberswalde rim, which are similar 

source rocks to alluvium occurring in Holden crater fans. Relief 

inversion of distributary paleochannels occurred in both areas 

but is more pronounced at Eberswalde, possibly reflecting a 

greater thickness or finer caliber of interchannel fines in its 

lower-energy depositional environment. Finally, both craters 

record somewhat variable water levels. In northwestern Holden 

crater, some stratigraphically older alluvial fans have distal 

scarps, suggesting either base-level control on deposition or erosion of the fan toe at a lower lake level. The strongest evidence 

for deep lakes in Holden crater is the occurrence of LTLDs at 

least 150 m thick and at elevations up to about –2,000 m along 

the crater walls, 300 m above the floor. In Eberswalde, lateral 

migration of river channels suggests base-level stability over 

century timescales, although multiple lobes of the delta indicate somewhat variable lake levels over much longer periods 

(Bhattacharya and others, 2005; Pondrelli and others, 2008). 

The fan stratigraphy suggests an aggrading delta surface (Lewis 

and Aharonson, 2006). 

The development of alluvial fans at Holden and a delta 

at Eberswalde likely reflects differences in the topography of 

the contributing area, rather than any significant difference in 

the paleoclimate or hydrology between the two adjacent sites. 

Eberswalde crater had an ~5,000 km2

 contributing watershed 

(not including the crater), but it was a smaller basin, helping 

to maintain deep-standing water. The paleolake had an area of 

~400 km2 at approximately the –1,400 m contour, so runoff 

production from the contributing watershed was roughly 1/12 

the evaporation rate of the lake. Neither Eberswalde nor Holden 

craters overflowed. Holden crater’s internal contributing area 

was ~19,000 km2, and the lake area would be ~9,800 km2 at 
6

the –2,000 m contour. Maintaining a sizeable lake in Holden 

crater before the rim was breached would have required either 

a greater precipitation rate than that at Eberswalde, which 

seems unlikely, or substantial contributions from Uzboi Vallis 

or regional groundwater (Holden crater is the deepest large 

basin within 700 km). The Holden impact basin’s inner ring 

and the Holden crater rim defined Eberswalde crater’s external 

watershed, which had a lower gradient than Holden’s steep 

walls and may have delivered finer sediment to the west side of 

Eberswalde. 

Mass movements in the map area are restricted to the 

entrance breach where Uzboi Vallis enters Holden crater, but 

this outcrop is too small to show at map scale. Incision of the 

breach appears to have triggered slides that may have punctuated the water and sediment flux into the crater. The final slide 

postdates erosion of the breach and is not modified by fluvial 

erosion.

Amazonian Period

Development of large alluvial fans appears to have continued into the Amazonian Period (Grant and Wilson, 2011). 

The only other sizeable Amazonian unit in the map area is the 

dune field (Ad) just inside the central ring of Holden crater. 

This deposit formed by transport of sand from the north. Its 

abundance of barchan dunes reflects a limited sand supply. At 

higher resolution, dark-toned aeolian ripples and sand sheets are 

common in the map area, particularly in topographic lows or 

corridors between elevated outcrops.

Structural Features

Most of the visible faults in the area date to the Late 

Hesperian or Early Amazonian epochs. These include compressional wrinkle ridges, mostly in the terra unit (HNt), and 

Late Hesperian to Early Amazonian extensional faults or floor 

fractures that are concentrated in and around the Ladon impact 

basin floor. The wrinkle ridges are mostly circumferential to 

Tharsis, and some older graben structures (ones that predate the 

final pulse of fluvial activity in the Hesperian) are radial to it 

(Grant, 1987).

Chaotic terrains are discrete areas of severe structural 

disruption that are often but not always confined to basins. 

They are commonly interpreted as collapse features, and the 

source of the subsurface void space is not well understood. 

Sharp (1973) commented that processes involving ground ice 

were volumetrically inadequate to the scale of the features and 

found magmatic flows more satisfactory. Chapman and Tanaka 

(2002) envisioned an interaction between magma and ground 

ice in forming chaotic terrains. In the map area, we find that 

many chaotic knobs in Morava Valles are topographically positive relative to the channel floor that they disrupt. Similarly, 

floor-fractured areas in southeastern Holden crater and Ladon 

Vallis are uplifted relative to older channel floors and alluvial 

deposits. Volcanic intrusions may be the most viable means of 

generating the localized uplift and creating the void space in 

the subsurface needed for subsequent collapse. Impact basin 

structures may have facilitated both floor fracturing and chaotic 

terrain development by these intrusions (Schultz, 1976).

Conclusions

The geology of the map area is summarized as follows 

(table 1, figs. 4–6):

Pre-Noachian Period

1. The long-wavelength topography of the crustal 

dichotomy formed.

Noachian Period

1. Middle Noachian or older impacts of the informally 

named Ladon and superimposed Holden impact basins 

exposed mountainous unit rocks (Nm).

2. The visible cratering record dates back to approximately the Middle/Late Noachian transition, based on 

N(16) populations of fresh and degraded craters.

3. Slow landscape degradation continued throughout the 

Noachian period, forming the terra unit (HNt). Relief 

reduction and smoothing occurred as weathering outpaced fluvial and aeolian transport of the weathered 

debris. 

4. Starting approximately at (potentially before) the 

Noachian/Hesperian transition, overflow of Argyre 

basin sent a catastrophic flood down the Chryse 

trough, overflowing and breaching Holden and Ladon 

basins and incising Uzboi, Ladon, and Morava Valles, 

and possibly Mawrth and (or) part of Ares Valles to 

the north. Early flooded surfaces represent the channel 

1 unit (HNch1), whereas later flow coalesced into a 

single channel, the channel 2 unit (HNch2).

Hesperian Period

1. Degraded craters and the Holden impact basin (units 

HNb1 and HNb2) accumulated fill materials during 

the Noachian period and into the Early Hesperian 

epoch. The Ladon basin floor (HNb2, Hb3) accumulated older sediments from the Uzboi, Ladon, and 

Morava system, later fluvial deposits from centripetally draining valleys, and possibly volcanic flows, 

all ending around the time that crater degradation 

declined in the Early Hesperian epoch.

2. The Hesperian impacts of Holden and Ostrov craters 

(Hc1) resurfaced the southwestern part of the map 

area.

3. Light-toned, layered deposits accumulated in 

Eberswalde and Holden craters, as well as Uzboi and 

Ladon Valles (unit Hl). Development of alluvial fans 

(AHf2) may have begun around this time. 
7

4. Overflow of the Holden crater rim by a lake in Uzboi 

Vallis deposited bouldery flood deposits (Hf1) in 

Holden crater. Deposition of fine-grained stratified 

deposits inside the Holden crater inner ring (Hh) may 

have been contemporary.

Amazonian Period

1. Late Hesperian to Early Amazonian chaotic terrain 

development (AHct) reflects uplift and dense fracturing followed by partial collapse, likely due to volcanic 

intrusion. 

2. Alluvial fan development (AHf2) may have continued 

into the Amazonian period. 

3. Stripping of the layered unit (Hl) from parts of the 

Eberswalde and Holden crater floors exposed older, 

potentially impact-related materials at the base of the 

etched unit (He).

4. Fresh impact craters (AHc2) that superimpose 

Noachian units date to the Hesperian and Amazonian 

periods.

5. The dune field (Ad) in Holden crater formed by sand 

transport from the north side of the crater, and smaller 

aeolian sand sheets formed in many locations.

6. Aeolian degradation of friable sedimentary deposits continued, leaving uncratered surfaces on some 

Hesperian geologic units.

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10

N(x) Area (km2) Count Per 106 km2 ± Error Age

Map area, all fresh and degraded craters (fig. 4)

N(2) 362,008 407 1,124 56 LN

N(5) 151 417 34 LN

N(16) 40 110 17 MN (MN–LN)

Interpretation: Ladon and Holden impact basins, which predate all other map features, formed around the MN/LN transition 

or earlier.

Incision of ULM system, HNch1 and HNch2, superimposed fresh and degraded craters

N(1) 17,909 71 3,964 470 EH

N(2) 18 1,005 237 LN (LN–EH)

N(5) 5 279 125 LN (LN–EH)

Interpretation: The ULM system formed in the LN to EH. Uncertainty is higher due to possible inclusion of some Holden 

secondary craters in the count or exclusion of primaries from it.

Terra unit HNt and superimposed AHc2 (fresh craters, fig. 5)

N(1) 177,820 647 3,635 143 EH

N(2) 160 899 71 EH

N(5) 32 180 32 EH (LN–EH)

Interpretation: Crater degradation waned in the EH.

Ladon impact basin floor HNb2 and Hb3 only (no large fresh craters or their ejecta)

N(1) 26,118 89 3,408 361 EH (EH–LH)

N(2) 31 1,187 213 LN (LN–EH)

N(5)

Interpretation: The Ladon impact basin floor was resurfaced at about the same time as the ULM flood and the waning of

crater degradation.

Ladon impact basin floor HNb2, Hb3, and superimposed AHc2

N(1) 46,171 133 2,881 250 LH (EH–LH)

N(2) 44 953 144 EH (LN–EH)

N(5) 7 152 57 EH (LN–LH)

Interpretation: Large fresh craters resurfaced much of the Ladon basin floor, so ages determined for the entire basin floor are

somewhat lower. Probable Holden crater secondaries formed on both basin fill units in Ladon basin.

Holden crater Hc1 and superimposed AHc2 (fig. 6)

N(1) 47,563 95 1,997 205 EA (LH–EA)

N(2) 31 652 117 EH (EH–LH)

N(5) 5 105 47 LH (EH–EA)

Interpretation: Holden crater formed in the Hesperian Period.

Ostrov crater Hc1 and superimposed AHc2

N(1) 20,238 50 2,471 327 LH

N(2) 13 642 173 EH (EH–LH)

N(5) 4 198 99 EH (LN–LH)

Interpretation: Ostrov crater formed in the Hesperian Period. Superposition of Ostrov secondaries over Holden secondaries 

suggests that Ostrov is younger. Crater wall erosion and deposition of alluvial fans in Holden and Ostrov craters occurred in 

the Hesperian or later. Less significant degradation of other Hesperian craters in the map area occurred around this time.

Table 1. Crater counts for spatially extensive map units. Areas are for entire outcrop extents within the map region.

[N(x), number of craters greater than x km in diameter per 106 km2, ± square root of count, also normalized to 106 km2. Age: EA, Early 

Amazonian; EH, Early Hesperian; LH, Late Hesperian; LN, Late Noachian; MA, Middle Amazonian; MN, Middle Noachian. Epoch boundaries follow Werner and Tanaka (2011), based on Hartmann (2005) isochrons, for N(1), N(2), N(5), and N(16). Where error bars cross epoch 

boundaries, the age is followed by the possible range in parentheses, for example EH (EH–LH). ULM, Uzboi-Ladon-Morava outflow system]
11

N(x) Area (km2) Count Per 106 km2 ± Error Age

Chaotic terrain AHct at Morava Vallis

N(1) 5,220 9 1,719 573 EA (LH–EA)

N(2) 2 382 270 EA (EH–MA)

N(5) 1 191 191

Interpretation: Chaotic terrain formed near Morava Vallis in the LH to EA. Floor fractures crosscut EH to LH materials 

in Ladon impact basin, Ladon Vallis, and Holden crater, so the fractures may be contemporary with chaotic terrain development.

Table 1. Continued